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git/Documentation/git-init.txt
Martin Ågren ff233d8dda Documentation: mark `--object-format=sha256` as experimental 
After eff45daab8 ("repository: enable SHA-256 support by default",
2020-07-29), vanilla builds of Git enable the user to run, e.g.,

  git init --object-format=sha256

and hack away. This can be a good way to gain experience with the
SHA-256 world, e.g., to find bugs that

  GIT_TEST_DEFAULT_HASH=sha256 make test

doesn't spot.

But it really is a separate world: Such SHA-256 repos will live entirely
separate from the (by now fairly large) set of SHA-1 repos. Interacting
across the border is possible in principle, e.g., through "diff + apply"
(or "format-patch + am"), but even that has its limitations: Applying a
SHA-256 diff in a SHA-1 repo works in the simple case, but if you need
to resort to `-3`, you're out of luck.

Similarly, "push + pull" should work, but you really will be operating
mostly offset from the rest of the world. That might be ok by the time
you initialize your repository, and it might be ok for several months
after that, but there might come a day when you're starting to regret
your use of `git init --object-format=sha256` and have dug yourself into
a fairly deep hole.

There are currently topics in flight to document our data formats and
protocols regarding SHA-256 and in some cases (midx and commit-graph),
we're considering adjusting how the file formats indicate which object
format to use.

Wherever `--object-format` is mentioned in our documentation, let's make
it clear that using it with "sha256" is experimental. If we later need
to explain why we can't handle data we generated back in 2020, we can
always point to this paragraph we're adding here.

By "include::"-ing a small blurb, we should be able to be consistent
throughout the documentation and can eventually gradually tone down the
severity of this text. One day, we might even use it to start phasing
out `--object-format=sha1`, but let's not get ahead of ourselves...

There's also `extensions.objectFormat`, but it's only mentioned three
times. Twice where we're adding this new disclaimer and in the third
spot we already have a "do not edit" warning. From there, interested
readers should eventually find this new one that we're adding here.

Because `GIT_DEFAULT_HASH` provides another entry point to this
functionality, document the experimental nature of it too.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2020-08-17 10:50:14 -07:00

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git-init(1)
===========
NAME
----
git-init - Create an empty Git repository or reinitialize an existing one
SYNOPSIS
--------
[verse]
'git init' [-q | --quiet] [--bare] [--template=<template_directory>]
[--separate-git-dir <git dir>] [--object-format=<format>]
[-b <branch-name> | --initial-branch=<branch-name>]
[--shared[=<permissions>]] [directory]
DESCRIPTION
-----------
This command creates an empty Git repository - basically a `.git`
directory with subdirectories for `objects`, `refs/heads`,
`refs/tags`, and template files. An initial `HEAD` file that
references the HEAD of the master branch is also created.
If the `$GIT_DIR` environment variable is set then it specifies a path
to use instead of `./.git` for the base of the repository.
If the object storage directory is specified via the
`$GIT_OBJECT_DIRECTORY` environment variable then the sha1 directories
are created underneath - otherwise the default `$GIT_DIR/objects`
directory is used.
Running 'git init' in an existing repository is safe. It will not
overwrite things that are already there. The primary reason for
rerunning 'git init' is to pick up newly added templates (or to move
the repository to another place if --separate-git-dir is given).
OPTIONS
-------
-q::
--quiet::
Only print error and warning messages; all other output will be suppressed.
--bare::
Create a bare repository. If `GIT_DIR` environment is not set, it is set to the
current working directory.
--object-format=<format>::
Specify the given object format (hash algorithm) for the repository. The valid
values are 'sha1' and (if enabled) 'sha256'. 'sha1' is the default.
+
include::object-format-disclaimer.txt[]
--template=<template_directory>::
Specify the directory from which templates will be used. (See the "TEMPLATE
DIRECTORY" section below.)
--separate-git-dir=<git dir>::
Instead of initializing the repository as a directory to either `$GIT_DIR` or
`./.git/`, create a text file there containing the path to the actual
repository. This file acts as filesystem-agnostic Git symbolic link to the
repository.
+
If this is reinitialization, the repository will be moved to the specified path.
-b <branch-name::
--initial-branch=<branch-name>::
Use the specified name for the initial branch in the newly created repository.
If not specified, fall back to the default name: `master`.
--shared[=(false|true|umask|group|all|world|everybody|0xxx)]::
Specify that the Git repository is to be shared amongst several users. This
allows users belonging to the same group to push into that
repository. When specified, the config variable "core.sharedRepository" is
set so that files and directories under `$GIT_DIR` are created with the
requested permissions. When not specified, Git will use permissions reported
by umask(2).
+
The option can have the following values, defaulting to 'group' if no value
is given:
+
--
'umask' (or 'false')::
Use permissions reported by umask(2). The default, when `--shared` is not
specified.
'group' (or 'true')::
Make the repository group-writable, (and g+sx, since the git group may be not
the primary group of all users). This is used to loosen the permissions of an
otherwise safe umask(2) value. Note that the umask still applies to the other
permission bits (e.g. if umask is '0022', using 'group' will not remove read
privileges from other (non-group) users). See '0xxx' for how to exactly specify
the repository permissions.
'all' (or 'world' or 'everybody')::
Same as 'group', but make the repository readable by all users.
'0xxx'::
'0xxx' is an octal number and each file will have mode '0xxx'. '0xxx' will
override users' umask(2) value (and not only loosen permissions as 'group' and
'all' does). '0640' will create a repository which is group-readable, but not
group-writable or accessible to others. '0660' will create a repo that is
readable and writable to the current user and group, but inaccessible to others.
--
By default, the configuration flag `receive.denyNonFastForwards` is enabled
in shared repositories, so that you cannot force a non fast-forwarding push
into it.
If you provide a 'directory', the command is run inside it. If this directory
does not exist, it will be created.
TEMPLATE DIRECTORY
------------------
Files and directories in the template directory whose name do not start with a
dot will be copied to the `$GIT_DIR` after it is created.
The template directory will be one of the following (in order):
- the argument given with the `--template` option;
- the contents of the `$GIT_TEMPLATE_DIR` environment variable;
- the `init.templateDir` configuration variable; or
- the default template directory: `/usr/share/git-core/templates`.
The default template directory includes some directory structure, suggested
"exclude patterns" (see linkgit:gitignore[5]), and sample hook files.
The sample hooks are all disabled by default. To enable one of the
sample hooks rename it by removing its `.sample` suffix.
See linkgit:githooks[5] for more general info on hook execution.
EXAMPLES
--------
Start a new Git repository for an existing code base::
+
----------------
$ cd /path/to/my/codebase
$ git init <1>
$ git add . <2>
$ git commit <3>
----------------
+
<1> Create a /path/to/my/codebase/.git directory.
<2> Add all existing files to the index.
<3> Record the pristine state as the first commit in the history.
GIT
---
Part of the linkgit:git[1] suite
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